The present invention relates generally to industrial process transmitters for use in industrial process control systems. More particularly, the present invention relates to process connections for coupling transmitters to high static pressure processes.
Process instruments are used to monitor process parameters, such as pressure, temperature, flow and level, of process fluids used in industrial processes. For example, process transmitters are typically employed in industrial manufacturing facilities at multiple locations to monitor a variety of process parameters along various production lines. Process transmitters include sensors that produce an electrical output in response to physical changes in the process parameter. For example, pressure transmitters include pressure transducers that produce an electrical output as a function of the pressure of a process fluid, such as in water lines, chemical tanks or the like. Each process transmitter also includes transmitter electronics for receiving and processing the electrical output of the sensor so that the transmitter and process parameter can be monitored locally or remotely. Locally monitored transmitters include displays, such as LCD screens, that show the electrical output at the site of the process transmitter. Remotely monitored transmitters include electronics that transmit the electrical output over a control loop or network to a central monitoring location such as a control room. Configured as such, the process parameter can be regulated from the control room by including automated switches, valves, pumps and other similar components in the process control system and the control loop.
Pressure sensors used in pressure transmitters include a flexible sensor element, such as an electrode plate or a piezo-resistor, that deflects in response to a pressure change. The sensor element is connected to the process fluid through a simple hydraulic system that communicates the process fluid pressure to the sensor. The hydraulic system comprises a sealed passageway in which the sensor element is positioned at a first end, and a flexible isolation diaphragm is positioned at a second end to engage the process fluid. The sealed passageway is filled with a precise amount of hydraulic fluid that adjusts the position of the sensor element as the process fluid influences the isolation diaphragm. As the pressure of the process fluid changes, the position of the sensor element changes, resulting in a change in capacitance or resistance of the pressure sensor. The electrical output of the pressure sensor is related to the capacitance or resistance and thus changes as the process fluid pressure changes. Differential pressure sensors typically require two hydraulic systems to bring separate pressures to opposite sides of a single sensor. Typically, a process flange having receptacles for process fluid lines is bolted to the base of the transmitter to align the process fluid with the isolation diaphragms of the differential hydraulic systems. Thus, the transmitter is coupled to the process in such a manner that the process fluid pressure is transmitted across the flange coupling.
Frequently it is desirable to connect transmitters to processes having extremely high static pressures. For example, deeply penetrating oil wells require large line pressures to transport the oil to surface levels. In particular, sub-sea oil wells require line pressures as high as approximately 15,000 pounds per square inch (psi) [˜103.4 Mega-Pascals (MPa)] to approximately 20,000 psi (˜137.9 MPa). The bolted connection between the flange and the transmitter base typically cannot seal at such high pressures due to stress limitations of the bolts and deformable seals used therebetween. Disruption of this seal causes process fluid to undesirably leak from the process transmitter coupling. Improvements to this seal are difficult in differential pressure transmitters where the isolation diaphragms are configured in an aligned-surface, or co-planar, arrangement within the base of the transmitter. As such, process transmitters are limited in their applicability by the pressures of the processes to which they can safely be coupled.